High-precision LED control circuit, method and LED driver thereof

ABSTRACT

In one embodiment, a light-emitting diode (LED) driver can include: (i) a reference voltage control circuit configured to provide a reference voltage signal in response to an enable signal; (ii) a current control circuit configured to control an output current of the LED driver in response to the reference voltage signal; and (iii) the LED driver being configured to drive an LED load when the enable signal is active.

RELATED APPLICATIONS

This application is a continuation of the following application, U.S.patent application Ser. No. 14/037,795, filed on Sep. 26, 2013, andwhich is hereby incorporated by reference as if it is set forth in fullin this specification, and which also claims the benefit of ChinesePatent Application No. 201210450521.8, filed on Nov. 12, 2012, which isincorporated herein by reference in its entirety.

FIELD OF THE INVENTION

The present invention relates to the field of light-emitting diode (LED)drivers, and more particularly to high-precision LED drivers andmethods.

BACKGROUND

With continuous innovation and development in the lighting industry, aswell as the increasing importance of energy-saving and environmentalprotection, LED lighting is becoming the prominent energy-efficientlighting technology. However, due to volt-ampere principles andtemperature characteristics, LEDs are more sensitive to current thanvoltage. Thus, conventional power supplies may not be directly providedto the LEDs. Rather, an appropriate LED driver can be employed with thepower supply when using LED as a lighting source.

SUMMARY

In one embodiment, a light-emitting diode (LED) driver can include: (i)a reference voltage control circuit configured to provide a referencevoltage signal in response to an enable signal; (ii) a current controlcircuit configured to control an output current of the LED driver inresponse to the reference voltage signal; and (iii) the LED driver beingconfigured to drive an LED load when the enable signal is active.

In one embodiment, a method of controlling an LED, can include: (i)providing, by a reference voltage control circuit, a reference voltagesignal in response to an enable signal; (ii) controlling, by a currentcontrol circuit, an output current of an LED driver in response to thereference voltage signal; and (iii) driving, by the LED driver, an LEDload when the enable signal is active.

Embodiments of the present invention can provide several advantages overconventional approaches, as may become readily apparent from thedetailed description of preferred embodiments below.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a schematic block diagram of an example LED driver.

FIG. 1B is a waveform diagram showing example operation of the LEDdriver of FIG. 1A.

FIG. 2A is a schematic block diagram of a first example control circuitfor an LED driver in accordance with embodiments of the presentinvention.

FIG. 2B is a waveform diagram showing example operation of the controlcircuit for the LED driver of FIG. 2A.

FIG. 3 is a schematic block diagram of a second example control circuitfor an LED driver in accordance with embodiments of the presentinvention.

FIG. 4 is a flow diagram of an example control method for an LED driverin accordance with embodiments of the present invention.

DETAILED DESCRIPTION

Reference may now be made in detail to particular embodiments of theinvention, examples of which are illustrated in the accompanyingdrawings. While the invention may be described in conjunction with thepreferred embodiments, it may be understood that they are not intendedto limit the invention to these embodiments. On the contrary, theinvention is intended to cover alternatives, modifications andequivalents that may be included within the spirit and scope of theinvention as defined by the appended claims. Furthermore, in thefollowing detailed description of the present invention, numerousspecific details are set forth in order to provide a thoroughunderstanding of the present invention. However, it may be readilyapparent to one skilled in the art that the present invention may bepracticed without these specific details. In other instances, well-knownmethods, procedures, processes, components, structures, and circuitshave not been described in detail so as not to unnecessarily obscureaspects of the present invention.

Referring now to FIG. 1A, shown is an example light-emitting diode (LED)driver. This example LED driver can be controlled by enable signal EN,which can be a square wave signal with a fixed duty cycle. When enablesignal EN is active, a power stage circuit can be enabled to transmitelectrical energy (e.g., voltage, current, etc.) from an input powersupply. When enable signal EN is inactive, the power stage circuit maybe disabled. In this way, e.g., LED dimming can be realized. As shown inFIG. 1A, control signal V_(ctrl) (e.g., output by a control circuit) andenable signal EN can be used to control main switch Q1 after a suitablelogic operation.

The transconductance amplifier in the control circuit of the LED drivercan receive reference voltage signal V_(ref) and output voltage feedbacksignal V_(fb). The transconductance amplifier can generate an outputcurrent for charging a capacitor based on a difference between referencevoltage signal V_(ref) and output voltage feedback signal V_(fb), so asto obtain compensation signal V_(C). In order to prevent compensationsignal V_(C) from rising continuously due to charging the capacitorthrough an output current of the transconductance amplifier when EN isinactive, an output of the transconductance amplifier can connect to aswitch that is controlled by enable signal EN. When the power stagecircuit is disabled, the switch can be turned off to stop the outputcurrent of the transconductance amplifier from charging the capacitor.

However, this particular circuit structure has some drawbacks, as shownin the example operation waveform diagram of FIG. 1B. From time t₀ totime t₂ when enable signal EN is high, output voltage feedback signalV_(fb) may gradually rise but may remain less than reference voltageV_(ref) prior to time t₁, and as a result compensation signal V_(C) mayalso gradually rise. From time t₁ to time t₂, output voltage feedbacksignal V_(fb) can be higher than reference voltage V_(ref), socompensation signal V_(C) may gradually decrease. At time t₂, the enablesignal may become inactive, so the switch at the transconductanceamplifier output can be turned off and compensation signal V_(C) maydrop to the voltage value as seen prior to time t₀.

From time t₂ to time t₃, output voltage feedback signal V_(fb) maygradually drop to zero. It can be seen that the integral value of outputvoltage feedback signal V_(fb) from time t₀ to time t₂ can besubstantially equal to a product of duty cycle D of enable signal EN andreference voltage signal V_(ref). Also, the integral value for a fulldimming cycle should also include the integral value from time t₂ totime t₃. During a dimming cycle, the integral values at the two inputterminals of the transconductance amplifier may not be equal to eachother. From time t₂ to time t₃, an output current formed by thedifference between output voltage feedback signal V_(fb) and referencevoltage V_(ref), as seen at the transconductance amplifier output maynot charge the capacitor. In other words, the variation information ofoutput voltage feedback signal V_(fb) may not be precisely representedby the value of compensation signal V_(C) during this period. As aresult, compensation signal V_(C) may be relatively small (e.g., inabsolute value, duty cycle, and/or duration), possibly leading toinaccurate LED load dimming. When the duty cycle of enable signal EN isrelatively low, this problem may be exacerbated.

In one embodiment, an LED driver can include: (i) a reference voltagecontrol circuit configured to provide a reference voltage signal inresponse to an enable signal; (ii) a current control circuit configuredto control an output current of the LED driver in response to thereference voltage signal; and (iii) the LED driver being configured todrive an LED load when the enable signal is active.

Referring now to FIG. 2A, shown is a schematic block diagram of a firstexample control circuit for an LED driver in accordance with embodimentsof the present invention. In this example, the LED driver can receiveenable signal EN, and can transform electrical energy (e.g., voltage,current, etc.) to drive an LED load when enable signal EN is active.Also, this particular LED driver can include reference voltage controlcircuit 201 and current control circuit 202. Reference voltage controlcircuit 201 can receive reference voltage signal V_(ref) and enablesignal EN. For example, reference voltage control circuit 201 canprovide reference voltage signal V_(ref) to current control circuit 202when enable signal EN is active.

Current control circuit 202 can include a compensation circuit formed bya transconductance amplifier and a compensation capacitor, as shown. Forexample, a non-inverting input of the transconductance amplifier canreceive reference voltage signal V_(ref) from reference voltage controlcircuit 201. Also, an inverting input of the transconductance amplifiercan receive output voltage feedback signal V_(fb) of the LED driver. Forexample, output voltage feedback signal V_(fb) can be derived from aseries connection of the LED load and a resistor, as shown. Of course,other arrangements and/or circuitry can be employed in order todetermine feedback information relative to the LED load and/or driver.Compensation signal V_(C) can be obtained based on the transconductanceamplifier output signal (e.g., a current) as passed through acompensation capacitor in order to form a compensation voltage. Currentcontrol circuit 202 can be utilized to control main switch Q1 to realizeelectrical energy transformation based on compensation signal V_(C).

For example, enable signal EN can include or be configured as a squarewave signal. In one example, the square wave signal may have a fixedduty cycle, and in other cases a variable duty cycle square wave can beemployed. Reference voltage control circuit 201 can include switches S₁and S₂. A first power terminal of switch S₁ can receive referencevoltage signal V_(ref), and a second power terminal of switch S₁ can beconfigured as an output of reference voltage control circuit 201. Thereference voltage control circuit output can connect to a non-invertinginput terminal of the transconductance amplifier, as shown. Also, acontrol terminal to control the switching operation of switch S₁ canconnect to enable signal EN. Enable signal EN can control operation ofswitch S₁. Switch S₂ can be coupled between an input of referencevoltage control circuit 201 and ground, and an inverted version ofenable signal EN can control operation of switch S₂.

For example, if enable signal EN is active high, reference voltagecontrol circuit 201 can output or provide reference voltage signalV_(ref) to current control circuit 202 (e.g., the compensation circuit)when enable signal EN is high. When enable signal EN is low, referencevoltage control circuit 201 can disallow reference voltage signalV_(ref) from being provided to current control circuit 202. Also, whenenable signal EN is low, the inverted version of enable signal EN ishigh, and switch S₂ can be closed in order to discharge the output ofreference voltage control circuit 201 to ground. Of course, otherarrangements or circuitry can be employed in order to provide a signalother than reference voltage signal V_(ref) when enable signal EN islow. For example, a different voltage level other than ground (e.g., adifferent reference level, a supply level, etc.) may be provided viaswitch S₂, or other multiplexer arrangements (e.g., two or more voltagelevels for selection) can also be supported in particular embodiments.

In the particular example of FIG. 2A, the power stage circuit of the LEDdriver can be configured as a boost converter. However, any suitableconverter topology (e.g., flyback, buck, Sepic, etc.) can be supportedin particular embodiments. In addition, different types of loads, suchas other lighting technologies, can also be supported. Further, othercircuit arrangements or implementations, such as for switching ormultiplex circuitry of reference voltage control circuit 201 and/oramplifier or capacitor configurations of current control circuit 202,can also be supported in certain embodiments.

Referring now to FIG. 2B, shown is a waveform diagram of exampleoperation of the control circuit for the LED driver of FIG. 2A. Fromtime t₀ to time t₁, enable signal EN may remain high, and referencevoltage signal V_(ref) can be provided to current control circuit 202.Thus, the transconductance amplifier can output a certain current tocharge the compensation capacitor based on a difference betweenreference voltage signal V_(ref) and output voltage feedback signalV_(fb). When the input signals of the transconductance amplifier arenearly equal (e.g., less than a predetermined amount), its outputcurrent can be almost zero, such that compensation signal V_(C) canremain substantially unchanged. At time t₁, as enable signal EN goeslow, the transconductance amplifier may not receive reference voltagesignal V_(ref), and may instead receive a ground level signal.

However, as output voltage feedback signal V_(fb) begins to drop, and anoutput of the transconductance amplifier is coupled with thecompensation capacitor, the compensation capacitor may begin todischarge. Thus, compensation signal V_(C) may gradually until time t₂when output voltage feedback signal V_(fb) drops to zero. It can be seenthat the integral value of output voltage feedback signal V_(fb) fromtime t₀ to t₂ can be substantially equal to the product of duty cycle Dof enable signal EN and reference voltage signal V_(ref). Therefore, theintegral values at the two inputs of the transconductance amplifier canbe substantially the same. When enable signal EN is inactive low,reference voltage signal V_(ref) may not be provided to current controlcircuit 202, and instead a ground level signal can be provided viaswitch S₂. However, current control circuit 202 may still ensure thatvariation information of output voltage feedback signal V_(fb) isaccurately represented by the value of compensation signal V_(C). Inthis way, more accurate LED load dimming can be realized, as compared toconventional approaches.

Referring now to FIG. 3, shown is a schematic block diagram of a secondexample control circuit for an LED driver in accordance with embodimentsof the present invention. In this particular example, current controlcircuit 202 can include a comparator and an RS flip-flop. Aftercomparing compensation signal V_(C) against a current sampling signalwith slope compensation, main switch Q1 can be turned off through the RSflip-flop. Main switch Q1 can be turned on by triggering the RSflip-flop through a clock signal (e.g., at the set input) with a certainor fixed frequency. Of course, other configurations and/or circuitry canbe employed for current control circuit 202.

The following will describe an example control method for an LED driverin accordance embodiments of the present invention. In one embodiment, amethod of controlling an LED, can include: (i) providing, by a referencevoltage control circuit, a reference voltage signal in response to anenable signal; (ii) controlling, by a current control circuit, an outputcurrent of an LED driver in response to the reference voltage signal;and (iii) driving, by the LED driver, an LED load when the enable signalis active.

Referring now to FIG. 4, shown is a flow diagram of an example controlmethod for an LED driver in accordance with embodiments of the presentinvention, which can include. At S401, a reference voltage signal can beprovided in response to an enable signal. For example, the referencevoltage signal can be provided (e.g., via reference voltage controlcircuit 201) when the enable signal is active, and a low signal (e.g.,ground) can be provided when the enable signal is inactive.

At S402, an output current can be controlled in response to thereference voltage signal. For example, control circuit 202 can beemployed in order to generate a compensation signal. In one case, thecompensation signal can be output in response to a comparison of thereference voltage signal and an output voltage feedback signal. At S403,the LED load can be driven when the enable signal is active. Forexample, main switch Q1 can be enabled via an output from an AND-gatethat receives an enable signal EN and a control signal V_(ctrl) outputfrom current control circuit 202. Also for example, the enable signalcan be configured as a square wave signal with fixed duty cycle.

In this way, a high-precision LED driver can be provided in particularembodiments. For example, the LED driver can include a power stagecircuit and a control circuit (e.g., including reference voltage controlcircuit 201 and current control circuit 202). The LED driver can receivean enable signal, and the power stage circuit can transform electricalenergy to drive an LED load when the enable signal is active. Thecontrol circuit can include any appropriate control circuit, includingvoltage and/or current control circuits, as described above.

The embodiments were chosen and described in order to best explain theprinciples of the invention and its practical applications, to therebyenable others skilled in the art to best utilize the invention andvarious embodiments with modifications as are suited to the particularuse contemplated. It is intended that the scope of the invention bedefined by the claims appended hereto and their equivalents.

What is claimed is:
 1. A light-emitting diode (LED) driver, comprising:a) a reference voltage control circuit configured to provide a referencevoltage signal in response to an enable signal, wherein said referencevoltage signal equals a first voltage value when said enable signal isactive, and equals a second voltage value when said enable signal isinactive, wherein said first voltage value is configured to represent anexpected output current of said LED driver; b) a current control circuitconfigured to generate a control signal to control an output current ofsaid LED driver in response to said reference voltage signal and afeedback signal that represents an output current of said LED driver;and c) a power switch controllable by said enable signal and saidcontrol signal, wherein said power switch is controlled by said controlsignal to drive an LED load when said enable signal is active, and saidpower switch is disabled when said enable signal is inactive.
 2. The LEDdriver of claim 1, wherein: a) said first voltage value is substantiallyequal to a voltage source when said enable signal is active; and b) saidsecond voltage value is substantially equal to zero when said enablesignal is inactive.
 3. The LED driver of claim 2, wherein said referencevoltage control circuit comprises: a) a first switch having a firstpower terminal configured to receive said voltage source, and a secondpower terminal configured to output said reference voltage signal whensaid enable signal is active; and b) a second switch coupled betweensaid second power terminal of said first switch and ground, wherein saidsecond switch is controllable by an inverted version of said enablesignal.
 4. The LED driver of claim 3, wherein said current controlcircuit comprises: a) a transconductance amplifier having a first inputterminal coupled to said second power terminal of said first switch, anda second input terminal coupled to said feedback signal; and b) acompensation circuit coupled to an output of said transconductanceamplifier, and configured to provide a compensation signal.
 5. The LEDdriver of claim 1, wherein said enable signal comprises a square wavesignal.
 6. The LED driver of claim 5, wherein said square wave signalcomprises a fixed duty cycle.
 7. The LED driver of claim 1, furthercomprising a power stage circuit comprising said power switch and beingconfigured to convert electrical energy to drive said LED load when saidenable signal is active.
 8. The LED driver of claim 1, wherein saidcurrent control circuit is configured to generate said control signal inaccordance with an error between said reference voltage signal and saidfeedback signal.
 9. A method of controlling a light-emitting diode(LED), the method comprising: a) providing, by a reference voltagecontrol circuit, a reference voltage signal in response to an enablesignal, wherein said reference voltage signal equals a first voltagevalue when said enable signal is active, and equals a second voltagevalue when said enable signal is inactive, wherein said first voltagevalue is configured to represent an expected output current of said LEDdriver; b) controlling, by a current control circuit generating acontrol signal, an output current of an LED driver in response to saidreference voltage signal and a feedback signal that represents an outputcurrent of said LED driver; and c) controlling a power switch by saidenable signal and said control signal, wherein said power switch iscontrolled by said control signal to drive an LED load when said enablesignal is active, and said power switch is disabled when said enablesignal is inactive.
 10. The method of claim 9, further comprisinggenerating a compensation signal in response to said reference voltagesignal and said feedback signal.
 11. The method of claim 9, wherein saidenable signal comprises a square wave signal.
 12. The method of claim 9,wherein said square wave signal comprises a fixed duty cycle.
 13. Themethod of claim 9, wherein said enable signal is an input-only signalreceived by said LED driver.
 14. The method of claim 10, wherein saidgenerating said control signal comprises comparing said compensationsignal against a current sampling signal with slope compensation. 15.The LED driver of claim 4, further comprising a comparator configured tocompare said compensation signal against a current sampling signal withslope compensation.
 16. The LED driver of claim 15, further comprisingan RS flip-flop having a set terminal coupled to a clock signal, a resetterminal coupled to an output of said comparator, and an output terminalconfigured as said control signal.
 17. The LED driver of claim 1,wherein said enable signal is an input-only signal received by said LEDdriver.